U.S. patent number 9,184,698 [Application Number 14/203,639] was granted by the patent office on 2015-11-10 for reference frequency from ambient light signal.
This patent grant is currently assigned to Google Inc.. The grantee listed for this patent is Google Inc.. Invention is credited to Robert Francis Wiser, Daniel James Yeager.
United States Patent |
9,184,698 |
Wiser , et al. |
November 10, 2015 |
Reference frequency from ambient light signal
Abstract
A system includes an oscillator referenced to a frequency
extracted from periodic intensity modulations of incident light.
The incident light can be intensity modulated based on the
frequency of the AC voltage that powers artificial lighting. The
system includes a light-sensitive element configured to generate an
output signal indicative of an intensity of incident light and a
controller. The controller can receive a first input signal based
on the output signal from the light-sensitive element. In the
presence of artificial lighting, the first input signal has a
frequency based on a reference frequency at which an intensity of
light incident on the light-sensitive element periodically varies.
The controller can generate a control signal based in part on the
reference frequency. The controller can provide the generated
control signal to the adjustable oscillator to thereby adjust the
oscillator frequency.
Inventors: |
Wiser; Robert Francis
(Cupertino, CA), Yeager; Daniel James (Berkeley, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Google Inc. |
Mountain View |
CA |
US |
|
|
Assignee: |
Google Inc. (Mountain View,
CA)
|
Family
ID: |
54363579 |
Appl.
No.: |
14/203,639 |
Filed: |
March 11, 2014 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H03B
28/00 (20130101); G02C 7/04 (20130101); H03B
17/00 (20130101); H03L 7/06 (20130101) |
Current International
Class: |
G02C
7/04 (20060101); H03B 28/00 (20060101); H03B
17/00 (20060101); H03L 7/06 (20060101) |
Field of
Search: |
;331/18,19,25,65,66,177R,187 ;351/158,159.01,159.02 |
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|
Primary Examiner: Gannon; Levi
Attorney, Agent or Firm: McDonnell Boehnen Hulbert &
Berghoff LLP
Claims
What is claimed is:
1. A system comprising: an adjustable oscillator configured to
generate a periodically varying output signal at an oscillator
frequency, wherein the oscillator frequency is at least partially
based on an input control signal; a light-sensitive element
configured to generate an output signal indicative of an intensity
of light incident on the light-sensitive element; a controller
configured to: (i) receive a first input signal based on the output
signal from the light-sensitive element, wherein the first input
signal has a frequency based on a reference frequency at which an
intensity of light incident on the light-sensitive element
periodically varies; (ii) generate a control signal based in part
on the reference frequency; and (iii) provide the generated control
signal to the adjustable oscillator to thereby adjust the
oscillator frequency; and a pulse generation circuit configured to
generate the first input signal received by the controller, wherein
the pulse generation circuit comprises a comparator configured to:
(i) receive a voltage signal based on the output signal from the
light-sensitive element; and (ii) output a series of voltage
pulses, wherein each voltage pulse corresponds to the received
voltage signal exceeding a threshold voltage.
2. The system of claim 1, wherein the controller is further
configured to determine a target multiple of the reference
frequency, and wherein the generated control signal is configured
to cause the oscillator frequency to become closer to the target
multiple of the reference frequency.
3. The system of claim 2, wherein the controller is further
configured to iteratively update the control signal such that the
oscillator frequency approaches the target multiple of the
reference frequency over time.
4. The system of claim 2, further comprising: a non-transitory data
storage; and wherein the controller is further configured to: (i)
while the light-sensitive element receives a periodically varying
light intensity indicative of the reference frequency, determine a
particular control signal based on the target multiple of the
reference frequency; (ii) store an indication of the particular
control signal in the non-transitory data storage; and (iii) while
the light-sensitive element does not receive a periodically varying
light intensity indicative of the reference frequency, use the
stored indication to generate the control signal.
5. The system of claim 1, wherein the controller is further
configured to: (i) receive a second input signal based on an output
signal from the adjustable oscillator, wherein the second input
signal has a frequency based on the oscillator frequency; (ii)
compare the first input signal and the second input signal; and
(iii) generate the control signal based in part on the comparison
between the first and second input signals.
6. The system of claim 5, further comprising: a counter configured
to: (i) count a number of cycles of the output signal of the
adjustable oscillator between successive cycles of the periodically
varying light intensity indicated by the first input signal, and
(ii) provide an indication of the counted number of cycles to the
controller; and wherein the controller is configured to generate
the control signal based in part on a comparison between the
counted number of cycles and a target number of cycles.
7. The system of claim 1, wherein the adjustable oscillator
comprises a voltage controlled oscillator.
8. The system of claim 1, wherein the light-sensitive element
comprises a photodiode.
9. The system of claim 1, further comprising: a polymeric material
formed to include a body-mountable surface; and a substrate at
least partially embedded within the polymeric material, wherein the
light-sensitive element, the adjustable oscillator, and the
controller are disposed on the substrate.
10. The system of claim 9, wherein the polymeric material has a
concave surface and a convex surface, wherein the concave surface
is configured to be removably mounted over a corneal surface and
the convex surface is configured to be compatible with eyelid
motion when the concave surface is so mounted.
11. The system of claim 1, further comprising: circuit components;
and a calibration system configured to receive the output signal
from the adjustable oscillator and use the oscillator frequency as
a timing reference to calibrate the circuit components.
12. A method comprising: generating, by a light-sensitive element,
an output signal indicative of an intensity of light incident on
the light-sensitive element; receiving, by a pulse generation
circuit comprising a comparator, a voltage signal based on the
output signal from the light-sensitive element and generating a
first controller-input signal comprising a series of voltage
pulses, wherein each voltage pulse corresponds to the received
voltage signal exceeding a threshold voltage, and wherein the first
controller-input signal has a frequency based on a reference
frequency at which an intensity of light incident on the
light-sensitive element periodically varies; receiving, by a
controller, the first controller-input signal and generating a
control signal based in part on the reference frequency; and
providing the generated control signal to an adjustable oscillator
to thereby adjust an oscillator frequency thereof.
13. The method of claim 12, further comprising: determining a
target multiple of the reference frequency; generating the control
signal such that the generated control signal is configured to
cause the oscillator frequency to become closer to the target
multiple of the reference frequency.
14. The method of claim 13, further comprising: iteratively
updating the control signal such that the oscillator frequency
approaches the target multiple of the reference frequency over
time.
15. The method of claim 13, further comprising: while the
light-sensitive element receives a periodically varying light
intensity indicative of the reference frequency, determining a
particular control signal value based on the target multiple of the
reference frequency; storing an indication of the particular
control signal in a non-transitory data storage; and while the
light-sensitive element does not receive a periodically varying
light intensity indicative of the reference frequency, using the
stored indication to generate the control signal.
16. The method of claim 12, further comprising: receiving, by the
controller, a second controller-input signal based on an output
signal from the adjustable oscillator, wherein the second
controller-input signal has a frequency based on the oscillator
frequency; comparing the first controller-input signal and the
second controller-input signal; and basing the control signal in
part on the comparison between the first and second
controller-input signals.
17. The method of claim 12, further comprising: using the
oscillator frequency as a timing reference to calibrate one or more
circuit components.
18. A body-mountable device comprising: a polymeric material formed
to include a body-mountable surface; a substrate at least partially
embedded within the polymeric material; an adjustable oscillator
disposed on the substrate, wherein the adjustable oscillator is
configured to generate a periodically varying output signal at an
oscillator frequency, wherein the oscillator frequency is at least
partially based on an input control signal; a light-sensitive
element disposed on the substrate, wherein the light-sensitive
element is configured to generate an output signal indicative of an
intensity of light incident on the light-sensitive element; a
controller disposed on the substrate, wherein the controller is
configured to: (i) receive a first input signal including a signal
generated based on the output signal from the light-sensitive
element, wherein the first input signal has a frequency based on a
reference frequency at which an intensity of light incident on the
light-sensitive element periodically varies; (ii) generate a
control signal based in part on the reference frequency; and (iii)
provide the generated control signal to the adjustable oscillator
to thereby adjust the oscillator frequency; and a pulse generation
circuit disposed on the substrate, wherein the pulse generation
circuit is configured to generate the first input signal received
by the controller, wherein the pulse generation circuit comprises a
comparator configured to: (i) receive a voltage signal based on the
output signal from the light-sensitive element; and (ii) output a
series of voltage pulses, wherein each voltage pulse corresponds to
the received voltage signal exceeding a threshold voltage.
19. The body-mountable device of claim 18, wherein the polymeric
material has a concave surface and a convex surface, wherein the
concave surface is configured to be removably mounted over a
corneal surface and the convex surface is configured to be
compatible with eyelid motion when the concave surface is so
mounted.
Description
BACKGROUND
Unless otherwise indicated herein, the materials described in this
section are not prior art to the claims in this application and are
not admitted to be prior art by inclusion in this section.
Artificial fluorescent or incandescent lighting is typically
modulated by the mains frequency of the alternating current (AC)
electrical power from the grid. Thus, in the US, lighting is
typically modulated at 120 Hertz (due to 60 Hertz utility waveform
reaching maximum voltage difference twice per cycle), and in
Europe, lighting is typically modulated at 100 Hertz (due to the 50
Hertz utility waveform).
In conventional electronics systems, individual electrical
components may vary due to variations in the fabrication process.
For instance, integrated circuit components may have different
values due to variations in the die process that forms the chip.
Such process variations can be accounted for, in practice, using at
least two precision components, which can be used as references to
calibrate the remaining components. One precision value can be
provided from the bandgap voltage of silicon. Another precision
value may be provided using an off-chip precision resistor that has
been separately calibrated. Using the combination of the silicon
bandgap voltage and the precision resistor value as references, the
remaining components on the chip can then be characterized and
calibrated for. In other examples, instead of a precision resistor,
a precision timing reference may be provided by a quartz oscillator
or another precision component connected to the chip. The precision
timing reference can then be used in combination with the silicon
bandgap voltage to calibrate other components.
SUMMARY
A system includes an oscillator referenced to a frequency extracted
from periodic intensity modulations of incident light. The incident
light can be intensity modulated based on the frequency of the AC
voltage that powers artificial lighting. The system includes a
light-sensitive element configured to generate an output signal
indicative of an intensity of incident light and a controller. The
controller can receive a first input signal based on the output
signal from the light-sensitive element. In the presence of
artificial lighting, the first input signal has a frequency based
on a reference frequency at which an intensity of light incident on
the light-sensitive element periodically varies. The controller can
generate a control signal based in part on the reference frequency.
The controller can provide the generated control signal to the
adjustable oscillator to thereby adjust the oscillator
frequency.
Some embodiments of the present disclosure provide a system
including an adjustable oscillator, a light-sensitive element, and
a controller. The adjustable oscillator can be configured to
generate a periodically varying output signal at an oscillator
frequency. The oscillator frequency can be at least partially based
on an input control signal. The light-sensitive element can be
configured to generate an output signal indicative of an intensity
of light incident on the light-sensitive element. The controller
can be configured to receive a first input signal based on the
output signal from the light-sensitive element. The first input
signal can have a frequency based on a reference frequency at which
an intensity of light incident on the light-sensitive element
periodically varies. The controller can be configured to generate a
control signal based in part on the reference frequency; and
provide the generated control signal to the adjustable oscillator
to thereby adjust the oscillator frequency.
Some embodiments of the present disclosure provide a method. The
method can include receiving a first input signal based on an
output signal from a light-sensitive element. The first input
signal can have a frequency based on a reference frequency at which
an intensity of light incident on the light-sensitive element
periodically varies. The method can include generating a control
signal based in part on the reference frequency. The method can
include providing the generated control signal to an adjustable
oscillator to thereby adjust an oscillator frequency thereof.
Some embodiments of the present disclosure provide a body-mountable
device including a polymeric material, a substrate, an adjustable
oscillator, a light-sensitive element, and a controller. The
polymeric material can be formed to include a body-mountable
surface. The substrate can be at least partially embedded within
the polymeric material. The adjustable oscillator can be disposed
on the substrate. The adjustable oscillator can be configured to
generate a periodically varying output signal at an oscillator
frequency. The oscillator frequency can be at least partially based
on an input control signal. The light-sensitive element can be
disposed on the substrate. The light-sensitive element can be
configured to generate an output signal indicative of an intensity
of light incident on the light-sensitive element. The controller
can be disposed on the substrate. The controller can be configured
to receive a first input signal including a signal generated based
on the output signal from the light-sensitive element. The first
input signal can have a frequency based on a reference frequency at
which an intensity of light incident on the light-sensitive element
periodically varies. The controller can be configured to generate a
control signal based in part on the reference frequency; and
provide the generated control signal to the adjustable oscillator
to thereby adjust the oscillator frequency.
Some embodiments of the present disclosure provide means for
receiving a first input signal based on an output signal from a
light-sensitive element. The first input signal can have a
frequency based on a reference frequency at which an intensity of
light incident on the light-sensitive element periodically varies.
Some embodiments of the present disclosure provide means for
generating a control signal based in part on the reference
frequency. Some embodiments of the present disclosure provide means
for providing the generated control signal to an adjustable
oscillator to thereby adjust an oscillator frequency thereof.
These as well as other aspects, advantages, and alternatives, will
become apparent to those of ordinary skill in the art by reading
the following detailed description, with reference where
appropriate to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an example system that includes a
body-mountable device in wireless communication with an external
reader.
FIG. 2A is a top view of an example eye-mountable device.
FIG. 2B is a side view of the example eye-mountable device shown in
FIG. 2A.
FIG. 3 is a functional block diagram of an example system
configured to provide a reference frequency based on incident
light.
FIG. 4 is a functional block diagram of an example system
configured to provide a reference frequency based on incident
light.
FIG. 5A is a functional block diagram of an example light detection
circuit for generating a series of pulses with a frequency based on
incident light.
FIG. 5B is a functional block diagram of an example light detection
circuit for generating a series of pulses with a frequency based on
incident light.
FIG. 6A is a functional block diagram of an example frequency
locking circuit.
FIG. 6B is a functional block diagram of an example frequency
locking circuit.
FIG. 7 is a flowchart of an example process for providing a
reference frequency based on a frequency of intensity modulation in
incident light.
FIG. 8 depicts a computer-readable medium configured according to
an example embodiment.
DETAILED DESCRIPTION
In the following detailed description, reference is made to the
accompanying figures, which form a part hereof. In the figures,
similar symbols typically identify similar components, unless
context dictates otherwise. The illustrative embodiments described
in the detailed description, figures, and claims are not meant to
be limiting. Other embodiments may be utilized, and other changes
may be made, without departing from the scope of the subject matter
presented herein. It will be readily understood that the aspects of
the present disclosure, as generally described herein, and
illustrated in the figures, can be arranged, substituted, combined,
separated, and designed in a wide variety of different
configurations, all of which are explicitly contemplated
herein.
I. Overview
An oscillator is referenced to an AC mains frequency using light
from an artificial light source. The artificial light source emits
light having a periodically varying intensity due to the AC voltage
that powers the lighting. Over the course of a cycle of the AC
waveform, the intensity of the emitted light reaches a maximum when
the magnitude of the AC voltage is maximized and the intensity
reaches a minimum when the magnitude of the AC voltage is
minimized. In a single cycle of a sinusoidal AC waveform, the AC
waveform has two zero crossings (one positive-headed zero-crossing
and one negative-headed zero-crossing) and also two magnitude
maximums (one at maximum positive voltage and one at minimum
negative voltage). The intensity of the emitted light is therefore
modulated at a frequency that is approximately twice the mains
frequency. Thus, artificial lighting has an intensity that varies
with a frequency of about 100 Hertz (for a 50 Hertz utility line
frequency) or about 120 Hertz (for a 60 Hertz utility line
frequency).
A system is disclosed herein that uses the periodically varying
light intensity from an artificial light source to calibrate an
oscillator frequency. The system includes a detection circuit with
a light-sensitive element, such as a photodiode, that responds to
incident light and provides an output signal indicating the
intensity of incident light. A comparator can then generate a
series of pulses at the frequency of the intensity modulation
(i.e., twice the mains frequency) by generating a pulse each time
the incident light exceeds a threshold, and that series of pulses
can be used to tune the frequency of the oscillator. For example,
the series of pulses can be used as an input to a frequency locked
loop, in which a controller adjusts an input to the adjustable
oscillator such that the oscillator frequency approaches a target
multiple of the frequency of intensity modulations over time.
To extract the frequency of the artificial lighting, the system
includes a light-sensitive element, an adjustable oscillator, and a
controller that adjusts the frequency of the oscillator based on an
output from the light-sensitive element. The light-sensitive
element can be a photodiode that provide a photocurrent current
related to the intensity of incident light. A transimpedance
amplifier can convert the photocurrent to a voltage, and the
voltage can be supplied to a comparator. The comparator can compare
the voltage from the transimpedance amplifier and identify
instances when the voltage exceeds a threshold, which instances can
correspond to relative maxima in the intensity modulated light
received at the photodiode. The comparator can output pulses for
each maxima of the received light intensity. The frequency of the
pulses correspond to the frequency of the intensity modulation
(i.e., twice the mains frequency). The pulses from the comparator
can then be used as a reference to adjust the frequency of the
adjustable oscillator.
The adjustable oscillator may be a voltage controlled oscillator
(VCO), and a controller or other feedback system can generate an
input to the VCO that adjusts the oscillator frequency. For
example, the controller can generate the input to the VCO based in
part on the comparator pulses so as to cause the VCO frequency to
achieve a target multiple of the frequency of the comparator
pulses. The controller may include one or more counters or other
logic that count the number of pulses output from the VCO between
successive pulses of the comparator. The number of pulses from the
VCO can then be compared to a target number of pulses, and the
input to the VCO can be increased or decreased based on the
comparison to tune the oscillator frequency to approach a target
multiple of the frequency of the intensity modulated light.
In one example, the VCO target frequency may be about 1.8 MHz. A
target frequency of 1.8 MHz corresponds to a target number between
each successive comparator pulse of about 15,000 (e.g., 1.8 MHz/120
Hz=15,000). For ambient light that is intensity modulated at a
frequency of 120 Hz (twice a 60 Hz AC mains line), the frequency
locked loop can tune the input to the VCO until the counter reaches
15,000 counts for each pulse from the comparator. If the counter
counts less than the target number, then the VCO is outputting a
signal with a lower frequency than the target value, and so the
input can be adjusted to increase the frequency of the VCO. If the
counter counts more than the target number, then the VCO is
outputting a signal with a greater frequency than the target value,
and so the input can be adjusted to decrease the frequency of the
VCO.
The frequency locked loop may include a digital to analog converter
for supplying the input to the VCO, and may also optionally include
non-volatile memory that stores an indication of a most-recently
supplied input. The stored indication can then be used in instances
when the system is not receiving intensity modulated light, such as
when the device is outdoors, in a dark environment, or otherwise
not in an environment illuminated by intensity modulated artificial
lighting. In some instances, the non-volatile memory may also store
more than one input, and vary the input to the VCO based on other
detected parameters known to alter the behavior of the VCO, such as
temperature.
Having such a calibrated oscillator may be useful in a variety of
applications. In some instances, the calibrated oscillator may be
used as a timing reference to calibrate and/or compensate for
process variations in other circuit components. In spatially
constrained and/or power constrained applications, such as
encountered in the context of body-mountable electronic devices,
including an off chip precision timing reference or a precision
resistor to use as a calibration reference may be impractical due
to the spatial and/or power constraints. As noted above, accounting
for process variations in the circuit components of an integrated
circuit can involve the use of two precision references, from which
other components can be calibrated. By calibrating an adjustable
oscillator using the intensity modulation of an artificial light
source, a precision timing reference can be obtained without
including a quartz oscillator or other off-chip precision
component.
One such body-mountable electronic device may be an eye-mountable
device formed of a polymeric material that is shaped to be contact
mounted to an eye, similar to a contact lens. A substrate embedded
within the polymeric material can be used to mount bio-interactive
electronics and associated power and communication electronics. In
one example, an antenna disposed on the substrate is used to
harvest energy from incident radio frequency radiation, and the
harvested energy can be used, via a rectifier and voltage
regulator, to power the remaining electronics. Communication
electronics can be used to modulate the impedance of the
energy-harvesting antenna to cause corresponding modifications of
the antenna's backscatter radiation, which can then be detected by
a reader.
II. Example Body-Mountable Electronics Platform
FIG. 1 is a block diagram of a system 100 that includes a
body-mountable device 110 in wireless communication with an
external reader 180. The exposed regions of the body-mountable
device 110 are made of a polymeric material 120 formed to be
contact-mounted to a corneal surface of an eye. A substrate 130 is
embedded in the polymeric material 120 to provide a mounting
surface for a power supply 140, a controller 150, sensor
electronics 160, and a communication antenna 170. The sensor
electronics 160 are operated by the controller 150. The power
supply 140 supplies operating voltages to the controller 150 and/or
the sensor electronics 160. The antenna 170 is operated by the
controller 150 to communicate information to and/or from the
body-mountable device 110. The antenna 170, the controller 150, the
power supply 140, and the sensor electronics 160 can all be
situated on the embedded substrate 130. Because the body-mountable
device 110 includes electronics and is configured to be
contact-mounted to an eye, it is also referred to herein as an
ophthalmic electronics platform.
To facilitate contact-mounting, the polymeric material 120 can have
a concave surface configured to adhere ("mount") to a moistened
corneal surface (e.g., by capillary forces with a tear film coating
the corneal surface). Additionally or alternatively, the device 110
can be adhered by a vacuum force between the corneal surface and
the polymeric material 120 due to the concave curvature. While
mounted with the concave surface against the eye, the
outward-facing surface of the polymeric material 120 can have a
convex curvature that is formed to not interfere with eye-lid
motion while the body-mountable device 110 is mounted to the eye.
For example, the polymeric material 120 can be a substantially
transparent curved polymeric disk shaped similarly to a vision
correction contact lens.
The polymeric material 120 can include one or more biocompatible
materials, such as those employed for use in contact lenses or
other ophthalmic applications involving direct contact with the
corneal surface. The polymeric material 120 can optionally be
formed in part from such biocompatible materials or can include an
outer coating with such biocompatible materials. The polymeric
material 120 can include materials configured to moisturize the
corneal surface, such as hydrogels and the like. In some
embodiments, the polymeric material 120 can be a deformable
("non-rigid") material to enhance wearer comfort. In some
embodiments, the polymeric material 120 can be shaped to provide a
predetermined, vision-correcting optical power, such as can be
provided by a contact lens. Moreover, the polymeric material 120
may be formed to facilitate mounting to another body surface, such
as a tooth surface, ear surface, skin surface, etc., and the
polymeric material 120 may have properties (e.g., flexibility,
surface hardness, lubricity, etc.) selected to be suitable for such
mounting locations.
The substrate 130 includes one or more surfaces suitable for
mounting the sensor electronics 160, the controller 150, the power
supply 140, and the antenna 170. The substrate 130 can be employed
both as a mounting platform for chip-based circuitry (e.g., by
flip-chip mounting to connection pads) and/or as a platform for
patterning conductive materials (e.g., gold, platinum, palladium,
titanium, copper, aluminum, silver, metals, other conductive
materials, combinations of these, etc.) to create electrodes,
interconnects, connection pads, antennae, etc. In some embodiments,
substantially transparent conductive materials (e.g., indium tin
oxide) can be patterned on the substrate 130 to form circuitry,
electrodes, etc. For example, the antenna 170 can be formed by
forming a pattern of gold or another conductive material on the
substrate 130 by deposition, photolithography, electroplating, etc.
Similarly, interconnects 151, 157 between the controller 150 and
the sensor electronics 160, and between the controller 150 and the
antenna 170, respectively, can be formed by depositing suitable
patterns of conductive materials on the substrate 130. A
combination of microfabrication techniques including, without
limitation, the use of photoresists, masks, deposition techniques,
and/or plating techniques can be employed to pattern materials on
the substrate 130.
The substrate 130 can be a relatively rigid material, such as
polyethylene terephthalate ("PET"), parylene, or another material
configured to structurally support the circuitry and/or chip-based
electronics within the polymeric material 120. The body-mountable
device 110 can alternatively be arranged with a group of
unconnected substrates rather than a single substrate. For example,
the controller 150 and a sensor in sensor electronics 160 can be
mounted to one substrate, while the antenna 170 is mounted to
another substrate and the two can be electrically connected via the
interconnects 157. In another example, the substrate 130 can
include separate partitions that each support separated, overlapped
coiled portions of the antenna 170. Such as, for example, an
example in which the antenna 170 is divided into multiple windings
that wrap around the body-mountable device 110 circumferentially at
respective radii, and are connected in parallel and/or in series.
To facilitate movement of the individual windings with respect to
one another, and thereby enhance flexibility of the body-mountable
device 110, and help prevent binding, etc., the individual windings
may each be mounted on divided portions of the substrate 130, which
may substantially correspond to the windings of such an
antenna.
The substrate 130 has a width sufficient to provide a mounting
platform for the embedded electronics components. The substrate 130
can have a thickness sufficiently small to allow the substrate 130
to be embedded in the polymeric material 120 without influencing
the profile of the body-mountable device 110. The substrate 130 can
have a thickness sufficiently large to provide structural stability
suitable for supporting the electronics mounted thereon. For
example, in an implementation in which the body-mountable device
110 is an eye-mountable device, the substrate 130 can be shaped as
a ring with a diameter of about 10 millimeters, a radial width of
about 1 millimeter (e.g., an outer radius 1 millimeter larger than
an inner radius), and a thickness of about 50 micrometers. The
substrate 130 can optionally be aligned with the curvature of an
eye-mountable surface of the polymeric material 120 (e.g., the
convex or concave surfaces). For example, the substrate 130 can be
shaped along the surface of an imaginary cone between two circular
segments that define an inner radius and an outer radius. In such
an example, the surface of the substrate 130 along the surface of
the imaginary cone defines an inclined surface that is
approximately aligned with the curvature of the eye mounting
surface (concave) and/or outward surface (convex) at that
radius.
The power supply 140 is configured to harvest ambient energy to
power the controller 150 and sensor electronics 160. For example, a
radio-frequency energy-harvesting antenna 142 can capture energy
from incident radio radiation. Additionally or alternatively, solar
cell(s) 144 ("photovoltaic cells") can capture energy from incoming
ultraviolet, visible, and/or infrared radiation. Furthermore, an
inertial power scavenging system can be included to capture energy
from ambient vibrations. The energy harvesting antenna 142 can
optionally be a dual-purpose antenna that is also used to
communicate information to the external reader 180. That is, the
functions of the communication antenna 170 and the energy
harvesting antenna 142 can be accomplished with the same physical
antenna.
A rectifier/regulator 146 can be used to condition the captured
energy to a stable DC supply voltage 141 that is supplied to the
controller 150. For example, the energy harvesting antenna 142 can
receive incident radio frequency radiation. Varying electrical
signals on the leads of the antenna 142 are output to the
rectifier/regulator 146. The rectifier/regulator 146 rectifies the
varying electrical signals to a DC voltage and regulates the
rectified DC voltage to a level suitable for operating the
controller 150. Additionally or alternatively, output voltage from
the solar cell(s) 144 can be regulated to a level suitable for
operating the controller 150. The rectifier/regulator 146 can
include one or more energy storage devices to mitigate high
frequency variations in the ambient energy gathering antenna 142
and/or solar cell(s) 144. For example, one or more energy storage
devices (e.g., a capacitor, a battery, etc.) can be connected in
parallel across the outputs of the rectifier 146 to regulate the DC
supply voltage 141 and configured to function as a low-pass
filter.
The controller 150 can be turned on when the DC supply voltage 141
is provided to the controller 150, and the logic in the controller
150 can then operate the sensor electronics 160 and the antenna
170. The controller 150 can include logic circuitry configured to
operate the sensor electronics 160 so as to sense a characteristic
of the environment of the body-mountable device 110. For example,
the sensor electronics 160 may include an analyte bio-sensor 162
configured to sense an analyte in the biological environment (e.g.,
tear film) of the body-mountable device 110. Additionally or
alternatively, the sensor electronics 160 could include a light
sensor 164 that is configured to detect an intensity of incident
light, or perhaps an image sensor configured to capture an image
from a perspective of the body-mountable device 110 (e.g., an
external environment outside of the eye or an internal environment
within the eye).
In one example, the controller 150 includes a bio-sensor interface
module 152 that is configured to operate analyte bio-sensor 162.
The analyte bio-sensor 162 can be, for example, an amperometric
electrochemical sensor that includes a working electrode and a
reference electrode. A voltage can be applied between the working
and reference electrodes to cause an analyte to undergo an
electrochemical reaction (e.g., a reduction and/or oxidation
reaction) at the working electrode. The electrochemical reaction
can generate an amperometric current that can be measured through
the working electrode. The amperometric current can be dependent on
the analyte concentration. Thus, the amount of the amperometric
current that is measured through the working electrode can provide
an indication of analyte concentration. In some embodiments, the
bio-sensor interface module 152 can be a potentiostat configured to
apply a voltage difference between working and reference electrodes
while measuring a current through the working electrode.
The controller 150 can include a light-referenced oscillator 154
that is calibrated based on an input from light sensor 164. The
light sensor 164 can include one or more photo-sensitive devices,
such as a photodiode. In some examples, the light sensor 164 is a
circuit including a photodiode that generates a photocurrent that
is proportionate to the intensity of incident light. In an
artificial lighting environment, the intensity is periodically
modulated based on a frequency of the AC mains that powers the
artificial lighting. The signal generated by the light sensor 164
in response to incident light includes an indication of the
periodic intensity modulation. The output from the light-sensor 164
can therefore be used to adjust the frequency of the oscillator 154
and thereby calibrate the frequency of the oscillator using the AC
mains frequency. The light-referenced oscillator may include an
adjustable oscillator, such as a voltage controlled oscillator. A
controller or other feedback loop can use an input signal from the
light sensor 164 (which indicates the frequency of intensity
modulated light) and adjust the frequency of the oscillator 154
based in part on that input signal.
Once calibrated, the light-referenced oscillator 154 can then be
used as a timing reference to calibrate other components in the
controller 150. Among other applications, the controller 150 may
include an integrated circuit formed by a die process, and various
components in the integrated circuit may have electrical properties
that deviate from ideal or intended values due to random variations
in the die process. Using the timing reference of the calibrated
light-referenced oscillator 154, and also the bandgap voltage of
silicon, the process variations can be accounted for. In some
cases, the two precision references (e.g., light-referenced
oscillator and silicon bandgap voltage) can be used to calibrate
and/or compensate for additional components included in the
controller 150.
The controller 150 can also include a communication circuit 156 for
sending and/or receiving information via the antenna 170. The
communication circuit 156 can optionally include one or more
oscillators, mixers, frequency injectors, etc. to modulate and/or
demodulate information on a carrier frequency to be transmitted
and/or received by the antenna 170. In some examples, the
body-mountable device 110 is configured to indicate an output from
a bio-sensor, light sensor, and/or image sensor by modulating an
impedance of the antenna 170 in a manner that can be perceived by
the external reader 180. For example, the communication circuit 156
can cause variations in the amplitude, phase, and/or frequency of
backscatter radiation from the antenna 170, and such variations can
be detected by the reader 180.
The controller 150 is connected to the sensor electronics 160 via
interconnects 151. For example, where the controller 150 includes
logic elements implemented in an integrated circuit to form the
bio-sensor interface module 152 and/or light sensor interface 154,
a patterned conductive material (e.g., gold, platinum, palladium,
titanium, copper, aluminum, silver, metals, combinations of these,
etc.) can connect a terminal on the chip to the sensor electronics
160. Similarly, the controller 150 is connected to the antenna 170
via interconnects 157.
It is noted that the block diagram shown in FIG. 1 is described in
connection with functional modules for convenience in description.
However, embodiments of the body-mountable device 110 can be
arranged with one or more of the functional modules ("sub-systems")
implemented in a single chip, integrated circuit, and/or physical
component. For example, while the rectifier/regulator 146 is
illustrated in the power supply block 140, the rectifier/regulator
146 can be implemented in a chip that also includes the logic
elements of the controller 150 and/or other features of the
embedded electronics in the body-mountable device 110. Thus, the DC
supply voltage 141 that is provided to the controller 150 from the
power supply 140 can be a supply voltage that is provided to
components on a chip by rectifier and/or regulator components
located on the same chip. That is, the functional blocks in FIG. 1
shown as the power supply block 140 and controller block 150 need
not be implemented as physically separated modules. Moreover, one
or more of the functional modules described in FIG. 1 can be
implemented by separately packaged chips electrically connected to
one another.
Additionally or alternatively, the energy harvesting antenna 142
and the communication antenna 170 can be implemented with the same
physical antenna. For example, a loop antenna can both harvest
incident radiation for power generation and communicate information
via backscatter radiation.
The external reader 180 includes an antenna 188 (or a group of
multiple antennas) to send and receive wireless signals 171 to and
from the body-mountable device 110. The external reader 180 also
includes a computing system with a processor 186 in communication
with a memory 182. The memory 182 is a non-transitory
computer-readable medium that can include, without limitation,
magnetic disks, optical disks, organic memory, and/or any other
volatile (e.g., RAM) or non-volatile (e.g., ROM) storage system
readable by the processor 186. The memory 182 can include a data
storage 183 to store indications of data, such as sensor readings
(e.g., from the analyte bio-sensor 162 and/or light sensor 164),
program settings (e.g., to adjust behavior of the body-mountable
device 110 and/or external reader 180), etc. The memory 182 can
also include program instructions 184 for execution by the
processor 186 to cause the external reader 180 to perform processes
specified by the instructions 184. For example, the program
instructions 184 can cause external reader 180 to communicate with
the body-mountable device 110. The program instructions 184 can
also cause the external reader 180 to provide a user interface that
allows for retrieving information communicated from the
body-mountable device 110 (e.g., sensor outputs from the analyte
bio-sensor 162 and/or light sensor 164). The external reader 180
can also include one or more hardware components for operating the
antenna 188 to send and receive the wireless signals 171 to and
from the body-mountable device 110. For example, oscillators,
frequency injectors, encoders, decoders, amplifiers, filters, etc.
can drive the antenna 188.
The external reader 180 can be a smart phone, digital assistant, or
other portable computing device with wireless connectivity
sufficient to provide the wireless communication link 171. The
external reader 180 can also be implemented as an antenna module
that can be plugged in to a portable computing device, such as in
an example where the communication link 171 operates at carrier
frequencies not commonly employed in portable computing devices. In
some instances, the external reader 180 is a special-purpose device
configured to be worn relatively near a wearer's eye to allow the
wireless communication link 171 to operate with a low power budget.
For example, the external reader 180 can be integrated in a piece
of jewelry such as a necklace, earing, etc. or integrated in an
article of clothing or an accessory worn near the head, such as a
hat, headband, a scarf, a pair of eyeglasses, etc.
In some embodiments, the system 100 can operate to non-continuously
("intermittently") supply energy to the body-mountable device 110
to power the controller 150 and sensor electronics 160. For
example, radio frequency radiation 171 can be supplied to power the
body-mountable device 110 long enough to operate the sensor
electronics 160 and communicate an outcome of such operation. In
such an example, the supplied radio frequency radiation 171 can be
considered an interrogation signal from the external reader 180 to
the body-mountable device 110 to request feedback (e.g., a sensor
measurement). By periodically interrogating the body-mountable
device 110 (e.g., by supplying radio frequency radiation 171 to
temporarily turn the device on), the external reader 180 can
accumulate a set of measurements (or other feedback) over time from
the sensor electronics 160 without continuously powering the
body-mountable device 110.
FIG. 2A is a top view of an example eye-mountable electronic device
210 (or ophthalmic electronics platform). FIG. 2B is an aspect view
of the example eye-mountable electronic device shown in FIG. 2A. It
is noted that relative dimensions in FIGS. 2A and 2B are not
necessarily to scale, but have been rendered for purposes of
explanation only in describing the arrangement of the example
eye-mountable electronic device 210. The eye-mountable device 210
is formed of a polymeric material 220 shaped as a curved disk. The
polymeric material 220 can be a substantially transparent material
to allow incident light to be transmitted to the eye while the
eye-mountable device 210 is mounted to the eye. The polymeric
material 220 can be a biocompatible material similar to those
employed to form vision correction and/or cosmetic contact lenses
in optometry, such as polyethylene terephthalate ("PET"),
polymethyl methacrylate ("PMMA"), polyhydroxyethylmethacrylate
("polyHEMA"), silicone hydrogels, combinations of these, etc. The
polymeric material 220 can be formed with one side having a concave
surface 226 suitable to fit over a corneal surface of an eye. The
opposite side of the disk can have a convex surface 224 that does
not interfere with eyelid motion while the eye-mountable device 210
is mounted to the eye. A circular outer side edge 228 connects the
concave surface 224 and convex surface 226.
The eye-mountable device 210 can have dimensions similar to a
vision correction and/or cosmetic contact lenses, such as a
diameter of approximately 1 centimeter, and a thickness of about
0.1 to about 0.5 millimeters. However, the diameter and thickness
values are provided for example purposes only. In some embodiments,
the dimensions of the eye-mountable device 210 can be selected
according to the size and/or shape of the corneal surface of the
wearer's eye and/or to accommodate one or more components embedded
in the polymeric material 220.
The polymeric material 220 can be formed with a curved shape in a
variety of ways. For example, techniques similar to those employed
to form vision-correction contact lenses, such as heat molding,
injection molding, spin casting, etc. can be employed to form the
polymeric material 220. While the eye-mountable device 210 is
mounted in an eye, the convex surface 224 faces outward to the
ambient environment while the concave surface 226 faces inward,
toward the corneal surface. The convex surface 224 can therefore be
considered an outer, top surface of the eye-mountable device 210
whereas the concave surface 226 can be considered an inner, bottom
surface. The "top" view shown in FIG. 2A is facing the convex
surface 224 From the top view shown in FIG. 2A, the outer periphery
222, near the outer circumference of the curved disk is curved to
extend into the page, whereas the central region 221, near the
center of the disk is curved to extend out of the page.
A substrate 230 is embedded in the polymeric material 220. The
substrate 230 can be embedded to be situated along the outer
periphery 222 of the polymeric material 220, away from the central
region 221. The substrate 230 does not interfere with vision
because it is too close to the eye to be in focus and is positioned
away from the central region 221 where incident light is
transmitted to the eye-sensing portions of the eye. Moreover, the
substrate 230 can be formed of a transparent material to further
mitigate effects on visual perception.
The substrate 230 can be shaped as a flat, circular ring (e.g., a
disk with a centered hole). The flat surface of the substrate 230
(e.g., along the radial width) is a platform for mounting
electronics such as chips (e.g., via flip-chip mounting) and for
patterning conductive materials (e.g., via microfabrication
techniques such as photolithography, deposition, plating, etc.) to
form electrodes, antenna(e), and/or interconnections. The substrate
230 and the polymeric material 220 can be approximately
cylindrically symmetric about a common central axis. The substrate
230 can have, for example, a diameter of about 10 millimeters, a
radial width of about 1 millimeter (e.g., an outer radius 1
millimeter greater than an inner radius), and a thickness of about
50 micrometers. However, these dimensions are provided for example
purposes only, and in no way limit the present disclosure. The
substrate 230 can be implemented in a variety of different form
factors, similar to the discussion of the substrate 130 in
connection with FIG. 1 above.
A loop antenna 270, controller 250, and sensor electronics 260 are
disposed on the embedded substrate 230. The controller 250 can be a
chip including logic elements configured to operate the sensor
electronics 260 and the loop antenna 270. The controller 250 is
electrically connected to the loop antenna 270 by interconnects 257
also situated on the substrate 230. Similarly, the controller 250
is electrically connected to the sensor electronics 260 by an
interconnect 251. The interconnects 251, 257, the loop antenna 270,
and any conductive electrodes (e.g., for an electrochemical analyte
sensor, etc.) can be formed from conductive materials patterned on
the substrate 230 by a process for precisely patterning such
materials, such as deposition, photolithography, etc. The
conductive materials patterned on the substrate 230 can be, for
example, gold, platinum, palladium, titanium, carbon, aluminum,
copper, silver, silver-chloride, conductors formed from noble
materials, other metals, combinations of these, etc.
The loop antenna 270 is a layer of conductive material patterned
along the flat surface of the substrate to form a flat conductive
ring. In some examples, to allow additional flexibility along the
curvature of the polymeric material, the loop antenna 270 can
include multiple substantially concentric sections electrically
joined together in parallel or in series. Each section can then
flex independently along the concave/convex curvature of the
eye-mountable device 210. In some examples, the loop antenna 270
can be formed without making a complete loop. For instances, the
antenna 270 can have a cutout to allow room for the controller 250
and sensor electronics 260, as illustrated in FIG. 2A. However, the
loop antenna 270 can also be arranged as a continuous strip of
conductive material that wraps entirely around the flat surface of
the substrate 230 one or more times. For example, a strip of
conductive material with multiple windings can be patterned on the
side of the substrate 230 opposite the controller 250 and sensor
electronics 260. Interconnects between the ends of such a wound
antenna (e.g., the antenna leads) can then be passed through the
substrate 230 to the controller 250.
When the eye-mountable device 210 is mounted over a corneal surface
of an eye, the motion of the eyelids distributes a tear film that
coats both the concave and convex surfaces 224, 226. The tear film
is an aqueous solution secreted by the lacrimal gland to protect
and lubricate the eye. The tear film layers coating the
eye-mountable device 210 can be about 10 micrometers in thickness
and together account for about 10 microliters. The eye-mountable
device 210 may allow for electrodes to be exposed to the tear film
via a channel in the polymeric material, or perhaps the polymeric
material may be configured to allow for diffusion of tear film
analytes to such sensor electrodes. For example, the sensor
electronics 260 may include one or more sensor electrodes of an
amperometric analyte sensor, and a channel in the outward-facing
convex surface 224 may expose the sensor electrodes to a layer of
tear fluid coating the convex surface 224. The sensor electronics
can then obtain an indication of an analyte concentration in the
tear film by applying a voltage to the sensor electrodes and
measuring a current through one or both of the sensor electrodes. A
suitable reagent can be fixed in the vicinity of the sensor
electrodes to facilitate an electrochemical reaction with a desired
analyte. As the analyte is consumed by such electrochemical
reactions, additional analyte diffuses to the sensor, and the rate
of re-supply (i.e., the rate at which the analyte diffuses to the
sensor) is related to the analyte concentration. The measured
amperometric current, which is related to the electrochemical
reaction rate, is therefore indicative of the analyte concentration
in the tear film.
III. Example Ambient Light Referenced Frequency Generation
Systems
FIG. 3 is a functional block diagram of an example system 300
configured to tune an oscillator frequency based on intensity
modulated incident light. The system 300 includes a detection
circuit 320 and a frequency locking circuit 330. An artificial
light source 310 illuminates the detection circuit 320 with light
312. The illuminating light 312 is intensity modulated at a
reference frequency f.sub.REF that is based on the mains frequency
of the electricity source that powers the artificial light source
310. For incandescent and fluorescent light sources, the luminosity
is related to the magnitude of the applied voltage, and so the
frequency of the intensity modulations is twice the AC mains
frequency. Although in some examples, the intensity modulated light
may have a frequency equal to the AC mains frequency. For example,
an artificial light source may include a polaritiy-dependent light
emitting device, such as a light emitting diode (LED) that emits
light when a positive polarity voltage is applied across the LED,
but not when the reverse polarity is applied. Other examples may
also be provided in which the light 312 is intensity modulated at
the AC mains frequency or twice the AC mains frequency. For
purposes herein, the frequency of the intensity modulations in the
light 312 will be referred to herein as the reference frequency
f.sub.REF.
The detection circuit 320 includes a light-sensitive element 322
that responds to the intensity modulated light 312, and a pulse
generation circuit 324 that generates an output signal 326 based on
an input from the light-sensitive element 322. The light-sensitive
element 322 may be a photodiode that generates a photocurrent
proportionate to the intensity of incident light. The pulse
generation circuit 324 receives an output from the light-sensitive
element 322 and generates a series of pulses having the reference
frequency f.sub.REF. The series of pulses from the detection
circuit 320 can then be provided to the frequency locking circuit
330 to use in tuning the oscillator frequency.
The frequency locking circuit 330 includes a controller 332 and an
adjustable oscillator 334. The adjustable oscillator 334 generates
an output signal 336 with a periodically varying waveform, such as
a series of pulses. The frequency of the periodically varying
output signal 336 can be adjusted based on an input from the
controller 332. For example, the adjustable oscillator 334 may
include a voltage controlled oscillator and the input may be a
voltage supplied via the controller 332 (e.g., using a digital to
analog converter). Changing the input voltage input to the voltage
controlled oscillator can thereby adjust the frequency of the
output signal 336. The controller 332 receives the signal 326 from
the detection circuit 320 (e.g., a series of pulses with frequency
f.sub.REF based on the intensity modulation of incident light). In
practice, the controller 332 can tune the input to the adjustable
oscillator 334 so as to cause the output 336 to have a frequency
that is a multiple of the frequency of the intensity modulations
(f.sub.REF).
To lock the frequency of the adjustable oscillator 334 to a target
multiple of f.sub.REF (i.e., a frequency N f.sub.REF) the
controller 332 may use the output signal 336 as feedback. The
controller 332 can compare the signal 326 indicative of the
reference frequency f.sub.REF and the output signal 336 and
generate the input based in part on the comparison. For example,
the controller 332 may include a counter or a similar device that
counts the pulses in the output signal 336 between one or more
successive pulses in the signal 326. The controller 332 may then
compare the counted number of pulses to the target multiple and
determine how to adjust the input to the adjustable oscillator 334
based in part on the comparison.
The controller 332 may be implemented using a combination of
hardware-implemented, software-implemented, and/or
firmware-implemented functional modules that coordinate to provide
the functionality described herein. In some cases, the controller
332 may include a data storage, which may be a non-transitory
computer readable medium on which data can be stored for later
retrieval. The data storage may store executable instructions that
can be executed by a processing system so as to perform functions
specified therein, for example. In some examples, the data storage
may also be used to store data indicative of particular control
inputs to provide to the adjustable oscillator 334 that cause the
system 300 to operate at a target multiple of the reference
frequency. In such examples, the controller 332 may use the stored
indication to subsequently generate the input to the adjustable
oscillator 334, such as in circumstances in which the system 300 is
not illuminated by artificial light, and the detector circuit 320
does not provide the input 326 indicative of the reference
frequency f.sub.REF. The stored indication may be based on a
most-recently undergone tuning of the adjustable oscillator 334 in
accordance with the light-based reference frequency f.sub.REF. In
addition, the data storage may also store data indicative of a
relationship between different control inputs and other measured
parameters that influence the adjustable oscillator 334, such as
temperature, for example. The controller 332 can then retrieve such
data from the data storage during a subsequent interval in which
intensity modulated light is not available, and generate the
control input based in part on the retrieved data.
FIG. 4 is a functional block diagram of an example system 400
configured to provide a reference frequency based on incident
light. The system 400 is similar in some respects to the system 300
described in connection with FIG. 3 and may be considered an
example implementation of the system 300. The system 400 includes a
detection circuit 420 and a frequency locking circuit 440. An
artificial light source 410 illuminates the detection circuit 420
with light 412 that is intensity modulated at a reference frequency
f.sub.REF.
The detection circuit 420 includes a photodiode 421, a
transimpedance amplifier 424, and a comparator 430. While the
photodiode 421 is reverse biased, the photodiode 421 generates a
photocurrent proportionate to the intensity of incident light. The
incident light creates electron-hole pairs in the light-sensitive
depletion region of the photodiode 421, and the reverse bias causes
the created electrons and holes to diffuse in opposite directions
to thereby generate an internal photocurrent directed from the
cathode to the anode. The photodiode 421 is reverse biased by a
bias voltage source 422, which applies a voltage to the cathode
greater than the voltage of the anode.
The transimpedance amplifier (TIA) 424 functions to convert the
current response of the photodiode 421 to a voltage signal. The TIA
424 includes an amplifier connected across the photodiode 421 to
provide a low impedance input for the photocurrent. A feedback
resistor 428 is arranged between the anode of the photodiode 421
and the output of the amplifier 426 and defines the gain of the
amplifier 426. The amplifier 426 can generate a voltage output
proportionate to the current through the photodiode 421. When the
photodiode 421 is illuminated by the intensity modulated light 412,
the voltage output from the TIA 424 is a voltage signal that varies
periodically at the frequency of the intensity modulation (e.g.,
the reference frequency f.sub.REF).
The voltage signal from the TIA 424 can be used to create a series
of pulses having a frequency based on the reference frequency
f.sub.REF (i.e., the frequency of the intensity modulations
detected with the photodiode 421). To cause the series of pulses to
have the same frequency as the intensity modulations, each pulse
can be generated in response to a periodically repeated feature in
the voltage signal from the TIA 424. As shown in FIG. 4, the pulses
may be generated using a comparator 430 that compares the voltage
signal of the TIA 424 with a threshold voltage 432. Upon the input
voltage (from the TIA 424) exceeding the threshold voltage 432, the
comparator 430 output goes high to initiate a pulse. And upon the
input voltage returning to below the threshold voltage 432, the
comparator output returns to low to halt the pulse. The threshold
voltage 430 is therefore selected to be between the maximum voltage
and minimum voltage of the periodically varying voltage signal from
the TIA 430. In some examples, the threshold voltage 432 may be
adjustable using a digital-to-analog converter or another
adjustable voltage source to allow for accommodating different
voltage ranges of the input voltage to the comparator 430. The
voltage from the TIA 424 varies based on the ambient light
intensity, and based on other factors. Adjustments to the threshold
voltage source 432 may therefore set the threshold voltage input to
the comparator 430 to be between the maximum and minimum voltages
of the periodically varying input voltage. By using an appropriate
value for the threshold voltage 432, each maximum of the intensity
modulated light 412 can correspond to a pulse in the output signal
434 from the detection circuit 420. For example, the threshold
voltage 432 may be set based on an average voltage of the
periodically varying voltage signal.
In addition, the net difference between maximum and minimum
voltages from the TIA 424 (e.g., corresponding to the magnitude of
the intensity variations) may depend on various factors, such as
the type of artificial light source 410. For example, the
instantaneous luminosity of an incandescent light source depends on
the temperature of the light-emitting filament that is heated by
the applied AC voltage from the mains line, and so the light
intensity varies periodically based on the periodic variation in
the applied AC voltage. In a fluorescent lighting system, the
fluorescent emission from the reaction tube is dependent on
accelerating electronics from an electrode to react with gas
substances in the tube. The electrons are accelerated in response
to the applied AC voltage, and so the emission rate varies
periodically based on the periodic variation in the applied AC
voltage. In either case, the instantaneous intensity of the emitted
light is correlated to some extent with the instantaneous magnitude
voltage of the AC waveform. However, the luminosity does not
completely turn off as the AC voltage passes through zero due to
the capacitance in the light source 410. For example, in an
incandescent light source, the heat capacity of the light-emitting
filament causes the filament to continue emitting light as the AC
voltage crosses zero. Depending on the heat capacity, the emitted
light may dim and brighten by different relative amounts throughout
the cycle of the applied AC voltage, and the relative difference in
the voltage signal from the TIA 424 (e.g., the amplitude of the
modulation) can also vary.
The frequency locking circuit 440 includes a first counter 442 and
a second counter 444 that provide inputs to a hardware logic module
446, and an adjustable oscillator 448. Based in part on the inputs
from the counters 442, 444, the logic module 446 generates a
control signal that is provided to the adjustable oscillator 448.
The counter 442 receives the signal 434 from the comparator 430
(e.g., a series of pulses at a reference frequency f.sub.REF). The
counter 442 increments a value for each pulse received from the
output signal 434, and provides an indication of the incremented
value to the logic module 446. Similarly, the counter 444
increments a value for each pulse received from the oscillator
output signal 450. The counter 444 can also provide an indication
of the current count of oscillator pulses, and may also be reset to
restart the count at zero. For example, the counter 444 may be
reset each time the counter 442 is incremented, and the number of
oscillator pulses between successive pulses of the signal 434 can
be indicated to the logic module 446 by the counter 444.
To achieve an output frequency that is a target multiple of the
reference frequency, the adjustable oscillator 448 is modified
using the control signal such that the number of oscillator pulses
counted between successive pulses at f.sub.REF (as indicated by the
signal 434 and counter 442. If the number counted by the counter
444 is less than the target multiple, then the logic module 446 can
tune the control signal to increase the frequency of the adjustable
oscillator 448. And if the number counted by the counter 444 is
greater than the target multiple, then the logic module 446 can
tune the control signal to decrease the frequency of the adjustable
oscillator 448.
In addition, the logic module 446 may integrate counts accumulated
by the counter 444 over multiple pulses at the reference frequency
f.sub.REF, as indicated by the counter 442, and tune the adjustable
oscillator 448 based on a comparison between the integrated count
(net count) reported by the counter 444 and a corresponding
multiple of the target multiple. By integrating over multiple
pulses of the reference frequency signal 434 output by the
detection circuit 420, the frequency locking circuit 440 may be
less susceptible to phase noise (e.g., timing jitter) in the signal
434. For example, the output signal 434 may exhibit phase noise due
to corresponding phase noise in the intensity modulations of the
received light 412 (e.g., due to underlying timing jitter in the AC
voltage). In addition, the pulse generation components may create
pulses with some additional phase noise due to imprecision in
firing the comparator 430 at precisely the same phase of the
underlying voltage waveform from the TIA 424. By integrating the
counter 444 over multiple cycles of the output signal 434, the
effect of phase noise from such effects is at least partially
mitigated by canceling out opposing timing jitter on different
pulses of the signal 434.
The system 400 includes one example of a light detection circuit
420 and a frequency locking circuit 440 that function together to
generate the oscillator output signal 450 at a target frequency
based on the reference frequency indicated by the intensity
modulated light 412 (e.g., a multiple of the reference frequency).
Alternative arrangements of the light detection circuit and/or
frequency locking loop can also be used to achieve a similar
result. Some additional examples of such alternative circuits are
described below in connection with FIGS. 5A-6B. To facilitate
comparisons between the different circuits, like components between
the circuits 420, 440 described in connection with FIG. 4 are
labeled with common element numbers in the following figures,
though it is to be understood that each disclosed circuit may
represent distinct embodiments. Further, some aspects of the
separate circuits may be combined in various ways.
3a) Example Light Detection Circuits
FIG. 5A is a functional block diagram of an example light detection
circuit 501 that is configured to generate a series of pulses with
a frequency based on incident light Like the light detection
circuit 420 discussed in connection with FIG. 4, the circuit 501
includes photodiode 421 biased by bias voltage 422 to operate in
photoconductive mode, and TIA 424 transforms the photocurrent
generated by the photodiode 421 to a voltage signal that is
provided to one input of comparator 430. However, rather than
comparing the output voltage signal from the TIA 424 to a threshold
voltage source, an analog filter 510 is coupled to the other input
of the comparator 430.
The analog filter 510 generates an input voltage to the comparator
430 for use in generating the series of pulses. The voltage
provided to the comparator 430 may therefore be referred to as a
comparison voltage. The comparison voltage may be, for example, a
DC bias of the varying voltage signal output from the TIA 424
and/or an average voltage of such varying voltage. In another
example, the comparison voltage is between the maximum and minimum
voltages in the output from the TIA 424, and may be adjusted over
time based on the series of pulses output by the comparator 430.
Similar to the description above, the comparator 430 outputs a
pulse for each occurrence of the varying voltage signal from the
TIA 424 crossing (i.e., exceeding) the comparison voltage (e.g.,
the voltage supplied by the analog filter 510). The analog filter
510 receives as input the series of pulses output from the
comparator 430, and therefore adjusts the comparison voltage based
on the series of pulses. While not specifically shown in FIG. 5A,
the analog filter 510 may additionally or alternatively receive an
input from the varying voltage output by the TIA 424, and use that
varying voltage to extract a DC bias (e.g., by low pass
filtering).
FIG. 5B is a functional block diagram of another example light
detection circuit 502 for generating a series of pulses with a
frequency based on incident light. Like the light detection circuit
420 discussed in connection with FIG. 4, the circuit 502 includes
the photodiode 421 biased by bias voltage 422 to operate in
photoconductive mode. And the TIA 424 transforms the photocurrent
generated by the photodiode 421 to a voltage signal that is
provided to one input of the comparator 430. However, rather than
comparing the output voltage signal from the TIA 424 to a threshold
voltage source, a digital-to-analog converter 522 is coupled to the
other input of the comparator 430. The digital-to-analog converter
522 provides a comparison voltage to the comparator based on
instructions from a logic module 520.
The logic module 520 receives as input the series of pulses output
from the comparator 430, and therefore adjusts the comparison
voltage (via the digital-to-analog converter 522) based on the
series of pulses output from the comparator 430. The logic module
520 may include, for example, a voltage sensor that monitors the
series of pulses. If such measurements indicate a degradation in
the series of pulses (e.g., noise) the logic module 520 may then
generate instructions to the digital-to-analog converter 522 to
either increase or decrease the comparison voltage to improve the
monitored characteristics of the series of pulses. While not
specifically shown in FIG. 5B, the logic module 520 may
additionally or alternatively receive an input from the varying
voltage output by the TIA 424, use a measurement of the varying
voltage to extract a DC bias, and instruct the digital-to-analog
converter 522 to provide a comparison voltage based on the
extracted DC bias.
3b) Example Frequency Locking Circuits
FIG. 6A is a functional block diagram of an example frequency
locking circuit 601. Like the frequency locking circuit 440
discussed in connection with FIG. 4, the circuit 601 includes
counters 442, a hardware logic module 446, and an adjustable
oscillator 448. The counter 442 receives a signal 434 from a light
detection circuit that includes an indication of the reference
frequency, such as a series of pulses repeated at the reference
frequency. The counter 442 increments a value upon receipt of each
periodic pulse (or other periodically repeated feature in the
signal 434). And the counter 442 provides an indication of the
number of pulses to the logic module 446. The logic module 446 can
then tune the control signal that adjusts the frequency of the
adjustable oscillator 448 based in part on the values provided by
the counter 442 in order to tune the adjustable oscillator 448 to a
target multiple of the reference frequency. However, rather than
comparing the number of counts of the oscillator frequency between
successive pulses at the reference frequency, as in FIG. 4, the
logic module 446 receives an input from a counter 612 that is
receiving the output of a divide-by-N module 610.
The divide-by-N module 610 receives the output signal 450 of the
adjustable oscillator 448 and outputs a corresponding pulse that
fires once for each N pulses of the oscillator output signal 450.
Thus, the divide-by-N module 610 translates the periodic output
signal 450 of the adjustable oscillator from the current oscillator
frequency (f.sub.OSC) to a periodic signal with a frequency
f.sub.OSC/N. If N is the target multiple of the reference frequency
f.sub.REF, then the logic module 446 functions to tune the
adjustable oscillator frequency (via the control signal) such that
the two counters 442, 612 are incremented at substantially the same
rate. If the counters 442, 612 are incremented at the same rate,
then the adjustable oscillator 448 has a frequency of f.sub.OSC=N
f.sub.REF. (i.e., the target multiple of the reference frequency).
And if the divide-by-N module 610 divides by a number other than
the target multiple of f.sub.REF, then the logic module 446 may
function to cause the two counters 442, 612 to increment at a
target rate with respect to one another that causes
f.sub.OSC.apprxeq.N f.sub.REF.
FIG. 6B is a functional block diagram of another example frequency
locking circuit 602. Like the frequency locking circuit 440
discussed in connection with FIG. 4, the circuit 602 includes a
hardware logic module 446 and an adjustable oscillator 448. The
logic module 446 receives an input from a phase detector 622 and/or
a phase-frequency detector. The phase detector 622 compares the
phases of two input signals, and indicates the relative phase
difference to the logic module 446, which the logic module 446 uses
to tune the adjustable oscillator 448.
One of the inputs to the phase detector 622 is based on the output
434 from a light detection circuit that includes an indication of
the reference frequency, such as a series of pulses repeated at the
reference frequency. As shown in FIG. 6B, the output signal 434 may
be provided to a divide-by-M module 620 which translates the
frequency of the signal 434 from f.sub.REF to f.sub.REF/M.
Alternatively, the divide-by-M module 620 may be omitted, and the
phase detector 622 may instead receive the output signal 434.
The other input to the phase detector 622 is based on the
oscillator output 450, and is therefore indicative of the present
oscillator frequency f.sub.OSC. As shown in FIG. 6B, the oscillator
signal 434 may be provided to a divide-by-N module 624 which
translates the frequency of the signal 434 from f.sub.OSC to
f.sub.OSC/N,
The logic module 446 can be used to minimize the phase difference
indicated by the phase detector 622 (e.g., by phase locking the
periodic variations in the two signals 434, 450) and thereby cause
the oscillator frequency f.sub.OSC of the oscillator output 450 of
adjustable oscillator 448 to approach a target multiple of the
output signal 434 indicative of the reference frequency f.sub.REF.
The target multiple of f.sub.REF may be based on the divide-by-N
module 624, the divide-by-M module 620, or both.
IV. Example Operations
FIG. 7 is a flowchart of an example process 700 for providing a
reference frequency based on a frequency of intensity modulation in
incident light. The process 700 may be performed by any of the
body-mountable devices described above that include a
light-referenced oscillator. Although it is noted that referencing
an oscillator to a frequency of intensity modulations in artificial
light may be used in a variety of different applications.
At block 702, intensity modulated light is received using a
light-sensitive element in a body-mountable device. For example,
the light may be received using a photodiode that generates a
photocurrent in proportion to the intensity of incident light. The
output from the photodiode varies periodically at a reference
frequency that is related to the AC mains frequency. Example light
detection circuits described in connection with FIGS. 3-5 include
examples of such light-sensitive circuits.
At block 704, a periodic signal, such as a series of pulses, is
generated with a frequency related to the intensity modulation of
the received light. For example, as described in connection with
FIG. 4, a series of pulses may be generated using a comparator that
compares a varying voltage based on an output from the light
sensitive element with a threshold voltage (e.g., the comparison
voltage described above). The comparator then outputs a pulse for
each occurrence of the varying voltage exceeding the threshold. In
some examples, the functions of blocks 702, 704 may be performed
using any of the light detection circuits described above.
At block 706, a control signal is generated to tune an adjustable
oscillator frequency based on the frequency of the series of
pulses. The control signal may be generated based on a comparison
between a number of oscillations (e.g., pulses) output from the
adjustable oscillator between successive pulses related to the
intensity modulated light, which occur at the reference frequency.
The counted number of oscillations can be compared with a target
number that corresponds to a target multiple of the reference
frequency. A controller, such as the logic module described herein,
may then generate the control signal provided to the adjustable
oscillator to increase or decrease the oscillator frequency such
that the oscillator frequency approaches the target multiple of the
reference frequency over time (and the number of oscillations
counted between successive pulses at the reference frequency
approaches the target number).
At block 708, the adjustable oscillator frequency is used as a
timing reference by components in the body-mountable device. The
calibrated oscillator may be used as a timing reference for a
variety of purposes. In one example, the circuitry components
within an integrated circuit may be calibrated and/or compensated
to account for process variations in such components by using the
timing reference. In one example, a precision voltage (e.g., a
silicon bandgap voltage) may be applied to a component, and then a
measurement can be made following a duration referenced using the
calibrated oscillator as a timing reference. The amount of
discharge during the known interval can then be used to determine
the electrical properties of the component (e.g., the capacitance,
resistance, etc.). As individual components become characterized in
that manner, additional components can be measured and calibrated
until the electrical properties of the integrated circuit are well
characterized.
In addition, some examples may include storing an indication of the
generated control signal used to achieve the target multiple of the
reference frequency (e.g., the most recently generated control
signal). For example, an indication of the control signal may be
stored in a non-transient data storage. Subsequently, the
controller that tunes the adjustable oscillator can retrieve the
stored indication and generate the control signal based on the
stored data. In some examples, the stored indication may be used in
instances when the device is not illuminated by an artificial light
source that is intensity modulated at a frequency based on the AC
mains frequency. Such controller may therefore first determine that
the device is not being illuminated by intensity modulated light
indicative of the reference frequency, and that determination may
be made based on an output from the light detection circuits
described herein. For example, if the frequency detection circuit
is unable to output a periodic series of pulses for a variety of
reasons, the frequency locking circuit may retrieve a stored
indication from data storage and generate a control signal to tune
the oscillator based on the retrieved data.
Moreover, it is particularly noted that while the electronics
platform is described herein by way of example as an eye-mountable
device or an ophthalmic device, it is noted that the disclosed
systems and techniques for small form factor imaging systems can be
applied in other contexts as well. For example, contexts in which
electronics platforms are operated with low power budgets (e.g.,
via harvested energy from radiated sources) or are constrained to
small form factors (e.g., implantable bio-sensors or other wearable
electronics platforms) may employ the systems and processes
described herein to reference an oscillator to a reference
frequency indicated by intensity modulations in received light. In
one example, an implantable medical device that includes a light
sensor and an adjustable oscillator may be encapsulated in
biocompatible material and implanted within a host organism. The
implantable medical device may include a circuit configured to
detect light received by a photo-sensitive element, extract a
reference frequency based on intensity modulations in the received
light, and tune an oscillator frequency based on the reference
frequency to thereby calibrate the oscillator. The configurations
disclosed herein allow for calibrating an oscillator frequency
based on incident light. The calibrated oscillator can then be used
as a timing reference for calibration and/or compensation of
process variations, for example. The present disclosure thereby
allows for inclusion of a timing reference in a spatially
constrained application in which another precision oscillator
(e.g., a quartz oscillator) is unsuitable.
FIG. 8 depicts a computer-readable medium configured according to
an example embodiment. In example embodiments, the example system
can include one or more processors, one or more forms of memory,
one or more input devices/interfaces, one or more output
devices/interfaces, and machine-readable instructions that when
executed by the one or more processors cause the system to carry
out the various functions, tasks, capabilities, etc., described
above.
As noted above, in some embodiments, the disclosed techniques can
be implemented by computer program instructions encoded on a
non-transitory computer-readable storage media in a
machine-readable format, or on other non-transitory media or
articles of manufacture (e.g., the instructions 184 stored on the
memory storage 182 of the external reader 180 of the system 100).
FIG. 8 is a schematic illustrating a conceptual partial view of an
example computer program product 800 that includes a computer
program for executing a computer process on a computing device,
arranged according to at least some embodiments presented
herein.
In one embodiment, the example computer program product 800 is
provided using a signal bearing medium 802. The signal bearing
medium 802 may include one or more programming instructions 804
that, when executed by one or more processors may provide
functionality or portions of the functionality described above with
respect to FIGS. 1-7. In some examples, the signal bearing medium
802 can be a non-transitory computer-readable medium 806, such as,
but not limited to, a hard disk drive, a Compact Disc (CD), a
Digital Video Disk (DVD), a digital tape, memory, etc. In some
implementations, the signal bearing medium 802 can be a computer
recordable medium 808, such as, but not limited to, memory,
read/write (R/W) CDs, R/W DVDs, etc. In some implementations, the
signal bearing medium 802 can be a communications medium 810, such
as, but not limited to, a digital and/or an analog communication
medium (e.g., a fiber optic cable, a waveguide, a wired
communications link, a wireless communication link, etc.). Thus,
for example, the signal bearing medium 802 can be conveyed by a
wireless form of the communications medium 810.
The one or more programming instructions 804 can be, for example,
computer executable and/or logic implemented instructions. In some
examples, a computing device such as the processor-equipped
external reader 180 of FIG. 1 is configured to provide various
operations, functions, or actions in response to the programming
instructions 804 conveyed to the computing device by one or more of
the computer readable medium 806, the computer recordable medium
808, and/or the communications medium 810.
The non-transitory computer readable medium 806 can also be
distributed among multiple data storage elements, which could be
remotely located from each other. The computing device that
executes some or all of the stored instructions could be an
external reader, such as the reader 180 illustrated in FIG. 1, or
another mobile computing platform, such as a smartphone, tablet
device, personal computer, etc. Alternatively, the computing device
that executes some or all of the stored instructions could be
remotely located computer system, such as a server.
While various aspects and embodiments have been disclosed herein,
other aspects and embodiments will be apparent to those skilled in
the art. The various aspects and embodiments disclosed herein are
for purposes of illustration and are not intended to be limiting,
with the true scope being indicated by the following claims.
* * * * *
References